It may be that properties are cutting back on these offers in the second half of the year.

It could also be that you are not booking sufficiently in advance. To take one of your examples, I have a MER booked in SF that week ... but I know that MER availability has been long gone. It's also the case that MER will sometimes show up for a two- or three-day stay but not for the individual days ... and vice versa!

I have to say that MERs have made this a "wonder year" for me -- I think I have had them on virtually all of my stays this year, and am on a glide path to making Diamond again as a result. Basic strategy has been to book in advance; to experiment with different combinations of days; to try and try again; and to be on the lookout for last-minute MERs when I have some flexibility.